Synthesis and Evaluation of Bifunctional [2.2.2]-Cryptands for Nuclear Medicine Applications

For the first time, synthesis of bifunctional [2.2.2]-cryptands (CRYPT) and demonstration of radiolabeling with lead(II) (Pb 2+ ) isotopes are disclosed herein. The synthesis is convenient and high-yielding and gives access to three distinct bifunctional handles (azide (−N 3 ), isothiocyanate (−NCS), and tetrazine (−Tz)) that can enable the construction of radioimmunoconjugates for targeted and pretargeted therapy. Proof-of-principle CRYPT radiolabeling was successful with lead-203 ([ 203 Pb]Pb 2+ ) and demonstrated complexation efficiency superior to that of DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and efficiency comparable to that of the current industry standard TCMC (1,4,7,10-tetraaza-1,4,7,10-tetra-(2-carbamoylmethyl)-cyclododecane). In vitro human serum stability assays demonstrated excellent [ 203 Pb]Pb-CRYPT stability over 72 h (91.7 ± 0.56%; n = 3). [ 203 Pb]Pb-CRYPT-radioimmunoconjugates were synthesized from the corresponding CRYPT-immunoconjugate or by conjugating [ 203 Pb]Pb-Tz-CRYPT to transcyclooctene modified trastuzumab (TCO-trastuzumab) via the inverse electron-demand Diels–Alder (IEEDA) reaction. This investigation reveals the potential for CRYPT ligands to become new industry standards for therapeutic and diagnostic radiometals in radiopharmaceutical elaboration.